The present invention relates generally to a technique for increasing the data rate without increasing the bandwidth requirement in a spread spectrum data link.
More particularly, the invention relates to an improvement (Next Generation Target Control System (NGTCS)) in tracking/ control systems for target aircraft, i.e. Drone Formation Control System (DFCS), Gulf Range Drone Control Upgrade System (GRDCUS), and Multiple Tracking and Control System (MTACS).
The NGTCS has a goal to remotely control 18 aircraft, with a growth potential of 24, by using a single data link frequency. These aircraft can be located over the horizon from the control facility, thus requiring a relay to retransmit the command messages. At present, the DFCS, GRDCUS, and MTACS data link waveform (hereinafter referred to as the "DFCS" waveform) can marginally support the 18 aircraft, over the horizon, scenario. In an effort to plan for growth potential, the basic data link waveform of these data links needs to be improved upon.
The following U.S. Pat. Nos. are of interest.
5,159,611--Tomita et al PA1 5,042,037--Endoh PA1 5,030,954--Ribnor PA1 5,029,185--Wei PA1 4,982,282--Saito et al PA1 4,972,506--Uddenfeldt PA1 3,875,332--Fletcher et al PA1 3,831,096--Brown, Jr.
The Tomita et al patent discloses a variable rate coder for transforming an input signal into a low bit rate digital signal. While this reference teaches the use of coding mean, such use is directed toward the achieving of a low bit rate in contrast to the objective of the present invention, i.e., increased data rate (each data pulse equals two bits of information. Endoh is directed to a digital data modulation circuit having a DC component suppression function. Endoh is similar to Tomira et al in his teaching that an object of his invention is to provide a circuit which operates in a manner that minimizes an increase in bit rate. Ribnor is concerned with improving resolution by doubling the oversampling ratio (ratio of initial to final conversion rates) without increasing the circuit speed requirements. While Ribnor teaches that resolution improves by 2.5 bits for each doubling of the oversampling ratio and increases 3.5 bits for a third-order modulator, there is no teaching of increasing data rate by coding the data in a manner that bandwidth requirement is not increased or that duty cycle is reduced. Wei is concerned with the use of coded modulation techniques in fading channel applications and notes the applicability of trellis-coded modulation thereto. While Wei points up the advantages of a "coded" modulation approach and the ultimate result of enhanced capability without requiring additional signal bandwidth, he does not address the selection of "another set of orthogonal chip codes" or the manner of coding such that the duty cycle is reduced. Saito et al, like Wei, is deficient in any teaching of the subject invention's selection or manner of coding, but is primarily concerned with an image signal compression encoding apparatus in which digital image data for a screen is subdivided into a plurality of blocks for the purpose of achieving a two-dimensional orthogonal transformation encoding on image data of each block. Uddenfeldt is directed to the solution of a problem with time-division multiple access (TDMA) type mobile radio systems. The solution, unlike the teaching of the subject invention, involves reducing the bit frequency of the data information flow. Fullerton is concerned with a time domain radio transmission system. Fullerton's invention clearly is directed to target identification (see col 2, lines 55-64). However, there is no teaching of coding in any particular manner to obtain increased data rate. The Fletcher reference concerns coded data rates of 1.0 mega bits per second but is limited to transmission lines that provide either dedicated or party line communications. Brown, Jr. clearly is concerned with tracking but is deficient in any teaching of increasing data rate by specific steps of the subject invention.